In Mongodb 7.0, I started getting extraordinarily long query times for some aggregate queries with $lookup. Namely, if the aggregate query was looking up against a collection using a field that had a sparse index:

document.student.createIndex(

{'teacher_id': 1}

, {
partialFilterExpression: {'teacher_id': {
$exists: true,
}},
});

db.teacher.aggregate([{$lookup: {from: 'student', let:

{teacherId: '$_id'}

, pipeline: [{'$match': {'$expr': {$eq: ['$teacher_id', '$$teacherId']}}}], as: 'students'}}]);

This is somewhat related to, but different from the following:
https://www.mongodb.com/community/forums/t/conditional-lookup/3833/10
https://jira.mongodb.org/browse/SERVER-44749